A piston for an internal-combustion engine (hereinafter, simply referred to as a “piston” in this Specification) performs a reciprocating motion repeatedly at a high speed under explosive pressure and high temperature conditions. Therefore, the piston is required to have a high strength while having low weight from a viewpoint of enhancing fuel consumption or the like.
In such a piston, an aluminum-silicon-based alloy such as AC8A is employed for reducing the weight thereof, and fine crystal grains in the internal-combustion engine piston is fine-grained by means of precipitation hardening treatment or the like for enhancing the strength thereof. However, in a gasoline engine, for example, a temperature of a piston sometimes increases to about 300° C., and when the piston whose strength is increased by means of the aforementioned precipitation hardening is used under high temperature conditions exceeding the temperature used in the precipitation hardening treatment (about 200° C. to 250° C.), the fine-grained crystals formed due to precipitation hardening are recrystallized thereby the crystal grains are coarsened, as a result, the strength of the piston considerably deteriorates.
Accordingly, there is a demand for a method of enhancing the high-temperature strength of an aluminum-silicon-based alloy that is used under high temperature conditions.
Although additive alloy components themselves are being reconsidered as a method of enhancing the high-temperature strength of such an aluminum-silicon-based alloy, for example, adding a large quantity of nickel copper or the like which is a component that enhances high-temperature strength will increase the weight of the piston due to an increase in the specific gravity of the aluminum-silicon-based alloy. Therefore, while it is possible to enhance the strength, it is impossible to satisfy the weight reduction requirement.
In addition, in the case of increasing the strength by reconsidering the composition of alloy components, it is difficult to form uniformly fine-grained alloy components during casting. As a result, there is a problem in that mechanical characteristics are not sufficiently improved and the quality is variable, and so on. Furthermore, enhancement of the material strength causes deterioration of casting and forging properties and workability on one hand, in particular, cutting workability is considerably deteriorated as the strength is increased, and thus, an inevitable problem that arises is the trade-off between strength enhancement and workability deterioration.
Accordingly, because the above-described increase in material strength causes deterioration in the production efficiency of the piston and an increase in the manufacturing costs, it is impossible to simply enhance the strength.
As above, when adjusting the amount of alloy material, because it is difficult to achieve both enhancement of high-temperature strength of the internal-combustion engine piston and weight reduction thereof without sacrificing production efficiency and workability, it has also been proposed to improve the mechanical characteristics by means of processing performed, ex post facto, on an aluminum alloy member manufactured through a casting and forging step (hereinafter, simply referred to as “aluminum alloy member) without modifying the processing performed in the casting and forging step.
As an example of such a method, a method for modifying a surface of an aluminum alloy member by applying shot peening treatment to the surface of the aluminum alloy member has been proposed. With the proposed the method for modifying the surface, by ejecting a mixture of shot materials and fine particles, shot peening is performed with fine particles included among the shot materials when a surface portion of the aluminum alloy member is bombarded with the shot materials, and thus, the above-described fine particles are dispersedly embedded in the surface portion of the aluminum alloy member (see Claim 1 of Patent Document 1). With this method, abrasion resistance and corrosion resistance are enhanced due to inherent properties possessed by the fine particles that are embedded by shot peening described above, and thus, the strength reliability of the aluminum alloy member is increased (see Paragraph[0017] of Patent Document 1).
In addition, in another proposed method, at a surface of a piston made of an aluminum-silicon-based alloy and obtained by casting and forging, particles to be ejected whose diameters are 20 μm to 400 μm and that contain a reinforcing element such as Fe, Mn, Zn, Ti, C, Si, Ni, Cr, W, Cu, Sn, Zr or the like which enhances the strength of the alloy by being diffused and penetrated in the alloy constituting the piston, are ejected toward and made to collide with the surface at an ejection velocity of 80 m/s or greater or at an ejection pressure of 0.3 MPa or greater; and, as a result of the collision with the particles to be ejected, oxides at surface defect portions generated in the surface of the piston due to the casting and forging are removed, the surface defects generated in the surface are repaired, the alloy elements in the alloy of the piston are fine-grained in the vicinity of the surface of the piston, the reinforcing element contained in the particles to be ejected is diffused and penetrated in the vicinity of the surface of the piston, and thus, a modified layer having a uniformly fine-grained metal structure which contains the alloy element and the reinforcing element contained in the particles to be ejected is formed at the surface of the piston (see Patent Document 2).